In current cancer treatment, the cure rate is reportedly about 50%, and healing is generally often achieved by topical therapies such as surgical therapy and radiotherapy. In particular, in the treatment of solid cancers, the relative contribution of chemotherapy, which is a systemic therapy, to healing when it is used alone, is very low, and it is common practice to use it in combination with various other therapies.
In surgical therapy, all visceral cancers are operable; this therapy is considered to have already reached completion as a therapeutic approach, and no further improvement in cure rate is expected. As for radiotherapy, therapeutic results for the treatment of responsive visceral organs have become nearly constant, and no further improvement in cure rate is expected, as with surgical therapy.
Therefore, because these therapies are no longer likely to significantly improve the cancer cure rate from now on, the cancer cure rate cannot be increased from the current level of 50% to achieve cancer control unless a better chemotherapy is developed.
Anticancer drugs used for chemotherapy are intended to obtain cell-killing effects on cells of high growth potential such as cancer cells, and cause major damage to normal cells, particularly to myelocytes and other cells of high cell growth potential, resulting in a major burden on the patient. This is because the anticancer drug is delivered by systemic administration of an injection, so that the anticancer drug reaches not only cancer cells but also normal cells and kills normal cells to hamper the functioning of homeostasis.
However, at present, the efficacy rate of an anticancer drug administered alone is reported to be roughly about 30%; although it is hoped that advances in analytical research in genetic information of genome will enable the selection of appropriate anticancer drugs in the future, the currently available anticancer drug therapy is reported to produce a higher prevalence of side-effects compared with their efficacy.
This is because normal cells are damaged by the systemic administration of the anticancer drug. Hence, provided that a method is established of introducing an anticancer drug specifically to cancer tissue, and incorporating the agent into cancer cells, an ideal anticancer drug delivery system would be realized. Furthermore, if it is possible to encapsulate an anticancer drug into a vesicle, a therapy could be established that act selectively on target organs or cells with little influence on normal cells (side-effects). Additionally, this is considered to lead to a re-evaluation of anticancer drugs the development of which has been discontinued due to strong side-effects.
Additionally, therapeutic results for malignant tumors have recently been remarkably improved with the advances in multidisciplinary treatment centered on chemotherapy. In particular, in hematopoietic tumors such as leukemia and malignant lymphoma, healing is well expectable when these therapies are used in combination with hematopoietic stem cell transplantation and the like. However, no more than toxic effects of anticancer drugs, radiotherapy and the like are observed in some cases; there is a limitation in the eradication of tumor cells. Basic research and clinical observations have shown that the selective elimination of tumor cells by immune system is important. The immune system acts by recognizing proteins (peptides) and non-peptide antigens such as saccharides and lipids, whether the relevant organ is mucosal or non-mucosal. Upon entry of pathogen in the body, mainly monocytes migrate to the entry site and exhibit antigen-nonspecific protective responses via phagocytosis and the like. Natural immunity against non-peptides such as saccharides and lipids and the like is first induced, helping the production of various factors concerning the elimination of pathogens. Subsequently, lymphocytes that recognize pathogenic peptides grow and differentiate; B lymphocytes differentiate into antibody-producing cells and T lymphocytes differentiate into helper T cells, cytotoxic T cells and the like, which control the immune system, thus inducing antigen-specific immune responses, i.e., what is called acquired immunity. There are two types of acquired immunity: humoral immunity, in which antibodies play the key role, and cellular immunity, in which T lymphocytes play the key role. Which is the prevalent type of immunity, whether cellular immunity or humoral immunity, depends on which is the prevalent subtype of helper T cells, whether Th1 or Th2. When the immune state is inclined to Th1-dominant, cellular immunity will prevail; when the immune state is inclined to Th2-dominant, humoral immunity will prevail. The two types of immunity occur in a mutual balance; these immune states rely on cytokines, which are humoral molecules secreted by various cells. As Th1 type cytokines, IL-12, IFNγ and the like can be mentioned; as Th2 type cytokines, IL-4, IL-5 and the like can be mentioned.
Considering immunity, particularly tumor immunity, it has been reported that CD8-positive cytotoxic T cells (CTL) and CD4-positive helper T cells play a very important role (North R J. 1984, Greenberg P D. 1991, Pardoll D M. 1998). In particular, it has been reported that CD8-positive T cells (CTL) of immunized animals directly injured target cells in vitro (Wanger H. 1980), and that tumor resistance was conferred to non-immunized animals by adoptive immunity (North R J. 1984, Greenberg P D. 1991). Therefore, how efficiently tumor-specific CTL can be induced is important in the development of antitumor therapy. In antitumor immunity with CTL, CTL recognizes a complex of a major histocompatibility complex (MHC) class I molecule and a tumor antigen-derived peptide, expressed on the tumor cell surface, via a T cell receptor and introduces perforin and the like into tumor cells, thereby exhibiting its cytotoxicity. In the induction of tumor-specific CTL, a focus is placed on first identifying a target antigen peptide that can be specifically expressed in tumor cells, processed in cells, and presented to MHC as a peptide fragment; many peptide molecules that produce high titers of IgG antibodies have been found by the serological analysis of recombinant cDNA expression library (SEREX) method.
However, it is difficult to describe tumor immunity solely based on the identification of a tumor peptide. Many challenges remain unresolved, including how efficiently to present the identified peptide on the cell surface in vivo, and the expression of CD80/CD86, which are co-stimulatory molecules. Speaking in detail, it has been reported that when the peptide is efficiently expressed, but the expression of CD80/CD86 and the like which are co-stimulatory molecules, is low, and the antigen signal alone is transmitted, growth of antigen-specific T cells does not occur and, what is more, T cell anergy develops in the cells that express the antigen (Gribben J G. 1996). In leukemia cells, many gene abnormalities have been shown to be involved in the mechanism for acquiring the growth dominance; it is considered that a specific abnormal protein formed due to such a gene abnormality is expressed, processed and fragmented, and the resulting peptide is presented to the groove of MHC on the cell surface. However, it has been reported to be difficult to induce an effective immune reaction to leukemia (Hirano N. 1996) because the expression of co-stimulatory molecules such as CD80/CD86 on the surface of many leukemia cells is insufficient despite the expression of such a leukemia-specific antigen thereby.
Also, as a recent finding, it has been reported that the reason why tumor rejection does not occur despite an increase of CTL is that the infiltration of immune cells and inflammatory cells is prevented by stroma cells present in the vicinity of the tumor (more than 90% of the tumor tissues of breast cancer, pancreatic cancer, and stomach cancer comprise interstitial fibroblasts), and that the tumor cell regression effect was dramatically increased by removing these disturbances (Yu P. 2004); the infiltration of inflammatory cells and immune cells in tumor cells has been considered to be an important factor concerning tumor regression.
Hence, successful utilization of tumor immunity relies on how efficiently tumor-specific CTL can be induced, and the following problems arise:                Whether or not any tumor-specific antigen has been identified?        What is to do if no tumor-specific antigen has not been identified?        How efficiently is an antigen presented to immunity-inducing cells (dendritic cells and the like)?        How efficiently is the maturation of immunity-inducing cells (dendritic cells and the like) achieved?        How to induce tumor immunity (mainly by CTL) with these points in mind        Efficient infiltration of immune cells in a tumor tissue        
Of the above-described problems, whether or not tumor immunity (mainly by CTL) is induced represents the most important requirement; an adjuvant that induces immunity is required for the induction of CTL. As an adjuvant capable of shifting the immune state to Th1-dominant, a patent application for “an adjuvant composed of HVJ-charged liposome” has been published (JP2001-302541A). Some reports are available on the effects thereof as a tumor vaccine [Anticancer Res., (19): 5367-5374. 1999, Ihshda H et al.; Hum. Gene. Ther., (10): 2719-2724. 1999, Zhou W Z. et al.; Gene. Ther., (6): 1768-1773. 1999. Zhou W Z. et al.; Mol. Ther., 5(3), 291-299, 2002. Tanaka M. et al.]. However, concerning the hemagglutinating virus of Japan envelope not in the form of liposome (hereinafter also referred to as HVJ-E), no such effects have been reported to date.
HVJ-E is a vector constructed on the basis of HVJ (JP2002-65278A); it permits the inclusion of a plasmid, oligo-DNA/RNA, protein, peptide, or low molecular compound in a vector vehicle, permits the introduction of the included sample into cells in the vicinity of the vector vehicle in vitro and in vivo, and enables the fusion of cells with each other by the action of the F protein, which is an envelope protein. The present inventors investigated tumor immunity using HVJ-E by making use of the above-described advantages.
Although immune gene therapy is the most suitable method of gene therapy for the metastasis suppression or recurrence prevention of cancer, its effect remains unsatisfactory worldwide. As a cause for this, it is postulated that tumor immunity cannot be enhanced to an extent sufficient to achieve successful treatment. To this end, we have been developing a gene transfer vector and a method of gene expression. As a result, it has become possible to administer an inactivated HVJ envelope as the adjuvant and an anticancer drug, that is encapsulated in the vector, or used in combination therewith, directly into a solid tumor, with coadministration of another anticancer drug, thereby inducing tumor-specific antitumor immunity. In this case, no tumor-specific antigen peptide is required; in other words, this is applicable to a broad range of tumors for which no tumor peptides have been identified. Furthermore, the present invention is groundbreaking in that the situation wherein CTL cells cannot reach the tumor site due to stroma cells and the like surrounding solid tumor tissue, despite induction of CTLs, and hence cannot kill the tumor cells of the solid tumor, so that no tumor regression is observed, can be overcome by using HVJ-E and an anticancer drug in combination.